This invention relates to a process for preparing (meth)acrylic acid. In particular, the present invention relates to a process for preparing (meth)acrylic acid which utilizes turndown control to maintain optimal distillation column performance in the dehydration of aqueous (meth)acrylic acid to provide crude (meth)acrylic acid.
Acrylic acid is generally prepared by the vapor phase catalytic oxidation of at least one hydrocarbon material. For instance, acrylic acid may be prepared from propylene and/or acrolein in a one or two step process. In a first step propylene is oxidized in the presence of oxygen, diluent inert gasses, water vapor, and appropriate catalysts to produce acrolein according to equation (I):
C3H6+O2xe2x86x92C2H3CHO+H2O+heatxe2x80x83xe2x80x83(I).
The acrolein is then oxidized, in a second step, in the presence of oxygen, diluent inert gasses, water vapor, and appropriate catalysts to form acrylic acid according to equation (II):
C2H3CHO+xc2xdO2xe2x86x92C2H3COOH+heatxe2x80x83xe2x80x83(II).
The acrolein may also be provided as starting material in a one step reaction (II) to produce acrylic acid.
Alternatively, propane may be used as a starting material. The propane is oxidized using appropriate catalysts, for instance, as described in U.S. Pat. No. 5,380,933 to form product acrylic acid.
Methacrylic acid is similarly prepared by the catalytic oxidation of isobutylene and/or isobutane.
The acrylic acid prepared using such vapor phase catalytic oxidation reactions is present in a mixed product gas exiting the reactor. Generally, the mixed product gas is cooled and is contacted with an aqueous stream in an absorption tower, thereby providing an aqueous acrylic acid solution which is then dehydrated in a distillation step to provide a crude acrylic acid stream. The crude acrylic acid stream can be used to produce various acrylic esters or be further purified to provide various grades of purified acrylic acid which can then be further utilized, for instance in the production of super absorbent polymer products or various other polymer materials.
Typically in the manufacture of acrylic acid there are instances wherein the aqueous acrylic acid feed rate to the distillation column and/or the aqueous acrylic acid feed stream composition fed to the distillation column changes. For instance, when the feed rate and/or composition of the aqueous acrylic acid feed stream is reduced below the maximum capacity of the system because of lower demand or as a result of variations in oxidation unit output. In a like manner, the feed rate of aqueous acrylic acid may increase as a result of increased demand or as a result of variations in the oxidation unit output. As a result, a changed amount or concentration of aqueous acrylic acid, is supplied to the distillation column, thereby leading to less or more aqueous acrylic acid being dehydrated in the distillation column and therefore the amount of crude acrylic acid produced is reduced or increased.
This can result in problems regarding maintaining optimal column performance, including adequate separation characteristics. Typically, at start up, a particular column is set to operate at a predetermined optimal vapor rate which is chosen to maintain suitable column performance. The optimal vapor rate is dependent on the distillation solvent to water ratio and distillation solvent feed rate for the system and is generally established by maintaining a predetermined distillation solvent to water ratio and distillation solvent feed rate. Also, a suitable amount of heat must be provided to the column to boil most of the distillation solvent and water overhead. Since it is generally assumed that the process will be operated at full capacity, an appropriate vapor rate is typically determined for optimal operation of the distillation column at or close to 100 percent of column capacity. Any changes from the predetermined capacity usage, i.e., changes in the aqueous acrylic acid feed or composition, may result in problems with the distillation column performance. For instance, when the feed rate of aqueous acrylic acid to a distillation column is reduced and the distillation solvent feed rate is held constant, the distillation column vapor rate is reduced and the distillation solvent to water ratio is increased. As a result, since the vapor rate is changed from its predetermined optimal value, column performance, including separation characteristics, suffers.
An additional problem may occur when product demand is reduced. Generally, if turndown control is unavailable, the manufacturer must completely shutdown to avoid excessive stockpiling of product acrylic acid which brings with it problems in storage, such as threat of polymerization, space usage and space availability. Furthermore, startup and shutdown procedures may be initiated more often.
As indicated above, the preparation and isolation of methacrylic acid proceeds by similar steps. Consequently, methacrylic acid manufacturers suffer from similar problems.
Distillation methods to remove water and impurities from aqueous (meth)acrylic acid solutions are know in the art. For instance, U.S. Pat. No. 5,785,821 discloses dehydration of an aqueous acrylic acid solution using a water insoluble solvent, e.g., toluene. The patent teaches wastewater recycle to the absorber of an acrylic acid process wherein the recycled wastewater stream has a specific composition of acetic acid (3-10 wt %), acrylic acid (0.5-5.0 wt%), and distillation solvent (0.01-0.5 wt%). Such a recycle stream, containing these specific amounts of acetic acid, acrylic acid and distillation solvent is said to enable collection of acrylic acid in the absorber at a high efficiency. However, this reference does not address the problem of distillation column turndown control in a process for preparing (meth)acrylic acid, which utilizes distillation to separate (meth)acrylic acid from water and impurities.
The present inventors have now discovered a process for preparing (meth)acrylic acid having turndown control in response to feed rate and/or composition changes in the aqueous (meth)acrylic acid stream fed to the distillation column. Such turndown control is achieved by controlling the amount of water or distillation solvent or both fed to the column to maintain a predetermined optimal vapor rate which provides optimal column performance. Furthermore, in one aspect, the invention enables obtaining additional value from process wastewater streams by utilizing such streams to adjust the water feed rate to the distillation column. This is done while maintaining suitable distillation column performance characteristics.
Accordingly, a novel process for preparing (meth)acrylic acid is described herein wherein the following advantages are provided:
(1) the ability to maintain column performance characteristics in response to feed rate and/or composition fluctuations in the aqueous (meth)acrylic acid fed to the distillation column, including the ability to avoid complete shutdown during low demand by running at reduced rates;
(2) additional value may be obtained from process wastewater streams by using them to adjust the water feed rate into the distillation column including recycling wastewater recovered from distillation column overheads directly for turndown control thereby reducing the wastewater load in the facility; and
(3) reducing yield loss in the distillation column overheads by recycling wastewater recovered from distillation column overheads directly for turndown control.
In one aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of (A) feeding an aqueous (meth)acrylic acid stream including (meth)acrylic acid to a distillation column; (B) distilling the aqueous (meth)acrylic acid stream, at a predetermined vapor rate, in the presence of at least one distillation solvent substantially insoluble in water, to form a crude (meth)acrylic acid stream, and (C) maintaining the predetermined vapor rate in response to aqueous (meth)acrylic acid fluctuation by (i) monitoring a distillation solvent to water ratio during distillation and (ii) adjusting at least one of the amount of water and the amount of distillation solvent fed to the distillation column to maintain the predetermined vapor rate.
In a second aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of (A) feeding an aqueous (meth)acrylic acid stream including (meth)acrylic acid to a distillation column; (B) distilling the aqueous (meth)acrylic acid stream, at a predetermined vapor rate, in the presence of at least one distillation solvent, substantially insoluble in water, to form a crude (meth)acrylic acid stream, and (C) maintaining the predetermined vapor rate in response to aqueous (meth)acrylic acid fluctuation by (i) monitoring a distillation solvent to water ratio during distillation and (ii) adjusting the amount of water fed to the distillation column to maintain the predetermined vapor rate, wherein at least a portion of the adjusting amount of water includes recycled waste water.
In a third aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of (A) feeding an aqueous (meth)acrylic acid stream including (meth)acrylic acid to a distillation column; (B) distilling the aqueous (meth)acrylic acid stream, at a predetermined vapor rate, in the presence of at least one distillation solvent, to form a crude (meth)acrylic acid stream, and (C) maintaining the predetermined vapor rate in response to aqueous (meth)acrylic acid fluctuation by (i) monitoring a distillation solvent to water ratio during distillation and (ii) adjusting at least one of the amount of water and the amount of distillation solvent fed to the distillation column to maintain the predetermined vapor rate.
In a fourth aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of (A) feeding an aqueous (meth)acrylic acid stream including (meth)acrylic acid and at least one polymerization inhibitor selected from 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy and derivatives thereof to a distillation column; (B) distilling the aqueous (meth)acrylic acid stream, at a predetermined vapor rate, in the presence of at least one distillation solvent, to form a crude (meth)acrylic acid stream, and (C) maintaining the predetermined vapor rate in response to aqueous (meth)acrylic acid fluctuation by (i) monitoring a distillation solvent to water ratio during distillation and (ii) adjusting at least one of the amount of water and the amount of distillation solvent fed to the distillation column to maintain the predetermined vapor rate.